Planar transformers are known whose power is limited to 2500 W at 300V, or to 1400 W at 2 kV.
The limiting of the power handled by a transformer involves using two to three converters each using a transformer in order to achieve a total power of 5 kW. A transformer capable of transferring 5 kW makes it possible to save on one to two converters.
The existing solutions are limited in power by:                the effects of proximity in the transformer limit either the usage frequency or the accessible copper section;        the thermal resistance of the transformer limits the power which can be dissipated in the transformer;        the high output voltage entails a significant electrical insulation which is accompanied by an increase in thermal resistance; and        the interleaving of the secondary and primary windings makes it possible to increase the frequency without reducing the copper section but also entails an increase in the electrical insulation layers which entails an increase in thermal resistance.        
FIG. 1 illustrates a planar transformer according to the prior art. The right-hand part of FIG. 1 shows the materials, and the left-hand part shows the heat fluxes.
Stacked individual windings 1, in this case three of them, are made up of several layers of copper 2, in this case two of them. These layers of copper or electrical conductors 2 are electrically insulated from one another by an insulator or dielectric 3. An insulating layer or dielectric layer is disposed between each of the individual windings 1, and between the individual winding 1 at the base of the stack and a cold source on which the stack of individual windings is disposed.
Cooling such a transformer through the magnetic core requires the heat dissipated in the conductors to pass through the dielectric layers which insulate the electrical conductors from one another and which insulate the conductors from the magnetic core. Since the dielectric materials are generally poor thermal conductors, the thermal resistance between the hot point of the conductors and the magnetic core is high (the thermal resistances of each dielectric layer are connected in series from the hot point to the magnetic core). Furthermore, since the magnetic core is also a source of heat dissipation, it does not represent a good cold source.
The use of the electrical connections as cold source makes it possible to cool the electrical conductors without passing through the series of dielectric layers. When the transformer is connected to a busbar, the heat can be removed by convection. When convection is not possible, the busbar is itself electrically insulated and does not therefore represent a good cold source.
An increase of the output voltage of such a transformer would entail increasing the thickness of insulation and consequently increasing the thermal resistance. The increase in thermal resistance would entail reducing the power transferrable through the transformer. To maintain the transferred power, it would be necessary to increase the volume and the weight of the transformer which would pose problems of resistance to the thermomechanical environment, which would lead an acceptable limit in terms of the weight and the volume of the current designs to be exceeded. Doubling the transferred power is therefore inconceivable with the known embodiments.
Furthermore, such a transformer has to operate in a vacuum which prevents the cooling by convection.